British Pat. No. 1363075 disclosed that strains of Streptomyces olivaceus are able to produce a useful .beta.-lactamase inhibiting substance. Belgian Pat. Nos. 827331 and 827332 showed that the material of British Pat. No. 1363075 was highly impure and contained only small quantities of antibacterially active agents. These antibacterially active agents were characterised in the Belgian Patents and were designated MM 4550 MM 13902. We have now found that in addition to MM 4550 and MM 13902, certain strains of Streptomyces olivaceus and related organisms produce a further antibacterially active .beta.-lactamase inhibiting compound. This new material is designated herein as MM 17880. Nothing herein should be construed as claiming any material disclosed in the aforementioned British of Belgian Patents or any process for the preparation of any such material. Other antibacterial agents are known to be produced by strains of Streptomyces, for example those disclosed in German Offenlegungsschrift No. 2340005 and that designated Thienamycin as disclosed in U.S. Pat. No. 3950357 and which was said to have the structural formula: ##STR1## Based on the present knowledge of the structure of MM 17880 significant structural differences are apparent between thienamycin and MM 17880.